Abstract

The surface waters of the Taupo region are of high quality and are sensitive to inputs of nitrogen. To reduce the amount of nitrogen discharged to surface water, the Taupo District Council (TDC) has employed a land treatment scheme (LTS), where treated municipal wastewater is irrigated onto ryegrass pasture. To limit the possibility of nitrogen pollution, regulations govern the amount of effluent that TDC may irrigate. This study reports the results from the first year of a five year trial where nitrogen leaching from the Taupo LTS was measured.

Effluent irrigation significantly increased pasture growth and nitrogen leaching compared to the un-irrigated treatments (P<0.001). The mean rate of pasture growth from the irrigated treatments was 15,800 ± 1,700 kg DM ha-¹yr-¹, but there were no significant difference between the rate of pasture growth between the irrigated treatments. The pasture of the high-N treatments had a significantly higher nitrogen concentration than the low-N treatments (P<0.001), consequently the high-N treatment removed 390 kg N ha-¹, compared to 310 kg N ha-¹ removed from the mid-N and low-N treatments. On average, the pasture removed 84 % of the nitrogen that was irrigated.

After 12 months, the no-N treatments leached 5 ± 3 kg TN ha-¹, the low-N treatment leached 15 ± 1 kg TN ha-¹, the mid-N treatment leached 17 ± 8 kg TN ha-¹, and the high-N treatment leached 26 ± 4 kg TN ha-¹. The high-N treatments leached significantly more TN than the low-N (P<0.005), but there was no significant difference in TN leached between the high-N and mid-N, or the mid-N and low-N treatments. The TN leached was poorly correlated with the rate of effluent irrigation. TN leached was positively correlated with the volume of water that drained through the soil (R2=0.7). The nitrogen in the leachate of the irrigated treatments comprised on average, 53 % NO₃-N, and 45 % DON, while the leachate of the un-irrigated treatments comprised, on average, 26 % NO₃-N and 72 % DON. NH₄-N accounted for approximately 2% of all nitrogen leached. Most of the NO₃-N leached throughout the year was leached after rain during summer and autumn. The mean concentration of NO₃-N leached from the irrigated treatments was 1.3 g N m-³. The concentration of NO₃-N in the leachate never exceeded Ministry of Health guidelines (11.3 g N m-³). The mean concentration of DON leached from the irrigated treatments was 1.2 g N m-³.

Removing nitrogen in the pasture is the solution to avoid excess nitrogen leaching from the Taupo LTS. There is potential to recover more nitrogen in the pasture by improving the pasture cover and frequency of harvest.